The production of crude oil from a formation involves a broad range of techniques and equipment. One such production technique is that of using a "down hole" pump submerged in a well containing formation fluid which is reciprocatively driven to lift the fluid through a tubing string to the well head from where it is piped to separation and storage facilities. Classically, a walking beam and more recently an improved hydraulic stroking device at the surface reciprocates the pump.
In a gas driven formation the formation fluid generally includes a mixture of water, oil, free gas and gas which has been forced into solution by formation pressure. As pumping of the well occurs, formation pressure may be reduced. Because of this reduced pressure, the flow rate of formation fluid into the well may become less than the rate at which the down hole pump can remove the fluid from the well. Consequently, eventually the hydrostatic head of the formation fluid in the well may fall to where a substantial quantity of the gas in solution is outgassed as the formation fluid enters the pump. This outgassed fluid may form a sizable pocket which gas locks the pump, i.e., the gas pocket cannot be compressed to the extent it can be pumped up the tubing string and the pressure in the pump cannot be lowered to a state wherein more fluid can be drawn into the pump to displace the gas pocket. When this occurs the well has become pumped off. A gas locked pump remains in that locked state until additional formation fluid flows into the well and the hydrostatic head of that fluid becomes sufficient to cause the fluid to enter the pump and displace the pocket of gas therein.
Traditionally, operators of wells wherein the pump apparatus can displace fluid more rapidly than formation fluid flows into the well have operated these apparatuses with alternate on-off cycles. For example, during a 24 hour period a pump apparatus may be operated for 4-8 hours and then shut down for the remainder of the period to permit the hydrostatic head of the formation fluid to build back up to where more fluid can be pumped. In many instances the wells become pumped off, i.e., pumps become gas locked, before the end of their operating periods because most operators time their pump apparatuses to run until liquid is no longer exhausted at the well head. It has been found that operating a pump in a gas locked state, in addition to being inefficient because no fluid is being pumped, causes excessive pump and seal wear and causes paraffin to deposit on the walls of the tubing string which may ultimately block the flow of fluid up that string. The occurrence of pump off in a well will be lessened if the flow of formation fluid into the well can be increased. It has been found that such flow can be increased if the back pressure in the well can be reduced. Thus, ideally a pump apparatus should be operated such that the hydrostatic head of the formation fluid will be kept as low as possible without gas locking the pump. Most conventional walking beam pumps cannot be operated in a manner such that the hydrostatic head of the formation fluid is kept at a minimum because it cannot be determined when they are gas locked. The occurrence of gas lock has been found to be reduced if the down hole pump is capable of pumping fluid having an low hydrostatc head and of outgassing small pockets of gas in the pumping chamber as shown in Applicant's copending U.S. patent application Ser. No. 732,850, filed May 10, 1985.
Although the flow of formation fluid into the well will be enhanced by a reduced back pressure in the well such a reduced pressure becomes undesirable during the pumping operation. During this operation a high back pressure becomes advantageous because it prevents outgassing of dissolved gases when formation fluid enters the pump. Thus, a high back pressure enables the pump to operate on fluid having a low hydrostatic head.
In view of the foregoing, it will be appreciated that the pumping efficiency of a gas-oil well which may become pumped off during the pumping operation will be enhanced greatly if a pumped off condition can be detected and a gas locked condition of the pump avoided and if the back pressure in the well can be reduced to increase the flow of formation fluid into the well without causing the pump to gas lock.